What's the effect of Contact lens Base Curve?

Discussion in 'Contact Lenses' started by Keith Morris, Jul 2, 2004.

  1. Keith Morris

    Keith Morris Guest

    I know what the base curve is and that it should closely match the curve of
    your cornea, but how does a doctor decide which to use? E.g. 8.6 vs 8.9mm
    seem really close -- how critical is it?

    And, how does an incorrect base curve affect comfort? I have one lens at
    8.6 that usually seems to feel "tight" -- would an 8.9 (flatter curve) be

    Keith Morris, Jul 2, 2004
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  2. Keith Morris

    Jan Guest


    A correct fitting is not only depending on a backcurve of a contactlens.
    Design of the backcurve(s) , diameter of the lens , among other decisions
    are to be made by a professional.
    You are wrong in assuming that a backcurve of a contactlens has to be as
    close as possible.
    Fittings issues can be discussed in general but a fitting done by internet
    directed to one persons eye is not possible.

    Jan (normally Dutch spoken)
    Jan, Jul 2, 2004
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  3. Keith Morris

    LarryDoc Guest

    The fit of a lens is dependent upon the relationship between the cornea
    curve and diameter, eye lid tension and thickness and then the lens base
    curve, diameter, edge shape and material structural rigidity. Not as
    simple as you might think.

    An 8.6 in one lens brand might be tight, another loose. Same for 8.9.
    Sometimes it's critical and sometimes not. Sometimes a "tight" lens
    feels better than a loose lens, while damaging the peripheral cornea and
    limbus. And sometimes not.


    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    larrydoc at eye-care-contacts dot com (remove -)
    LarryDoc, Jul 2, 2004
  4. Keith Morris

    Keith Morris Guest

    Okay, so how does one decide when to try a different base curve? Can you
    give an example of symptoms that would indicate incorrect base curve? I
    can't beleive a doctor can actually measure all the paramters you mentioned
    (e.g. eye lid tension and thickness) and "calculate" which base curve is
    correct -- he/she has to go by symtoms -- correct?

    Keith Morris, Jul 4, 2004
  5. Keith Morris

    LarryDoc Guest

    Knowing the eye parameters, lens parameters and patient's needs and
    desires allow us to choose "best choice" lenses for trial. Then we
    observe the lens on the eye and, based on objective observation and
    wearer subjective feedback, decisions can be made on how to fine-tune
    the lens fit.

    If it feels good and you see well, it's likely a good lens fit. If it
    hurts or you don't see well, then......well you can figure that out!
    Having a competent eyecare professional who knows how to look for things
    that might result in problems later on is the best approach. I think
    that's pretty CLEAR, yes?


    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    larrydoc at eye-care-contacts dot com (remove -)
    [website and email temporarily unavailable, probably until 7/5]
    LarryDoc, Jul 4, 2004
  6. Keith Morris

    Keith Morris Guest

    Okay -- so if you had a lens that produces a mild foreign-body sensation
    (tolerable but annoying -- and it sometimes goes away), and close-up vision
    comes and goes, and there are sometimes ghosts (of lights and other bright
    objects) -- would that say anything about the fit (I've tried swapping
    lenses, new lenses, etc, so its no a problem with a specific lens). My left
    eye is fine with the exact same lens -- it's the right one that is having
    the problem (and it didn't beofre when I had different lenses that were
    larger diameter. My own doc is out of ideas.

    Keith Morris, Jul 5, 2004
  7. Keith Morris

    LarryDoc Guest

    Well that's a pretty telling statement: you had a lens previously that
    worked and this new one does not. You answered your own question: it is
    either the incorrect power, fit or a design not compatible with your
    right eye. It is perfectly possible for the left to fit and the right


    Dr. Larry Bickford, O.D.
    Family Practice Eye Health & Vision Care

    The Eyecare Connection
    larrydoc at eye-care-contacts dot com (remove -)
    LarryDoc, Jul 5, 2004
  8. Keith Morris

    George Bray Guest

    I'm a bit confused to. Looking back at my notes, my prescription for
    standard lens (non-disposable - 40% water) nearly 20 years ago (adult
    age) was Base Curve 9.2 Diameter 15 Power -4.25

    A recent supply of 1 month disposable lens (55% water) was marked Base
    Curve 8.6 Diameter 14 Power -5.00

    OK, so I'm 20 years older and my sight has got even worse. (So much
    for people who think their sight is stable after about 21 years old
    and hve laser / lasik surgery!)

    But would you expect the curve of my cornea to alter as I get older?
    like from 9.2 to 8.6? Or could that change simply reflect the
    super-thin construction of the 1 month disposable lens?

    I should add that the doctor says my base curve hasn't changed and
    remains 8.9.
    Perhaps I'm just given what's in stock at any one time.

    The 8.6 lenses don't feel too bad. The 9.2 lenses also felt OK twenty
    years ago. Yet I had to visit a hospital on holiday back then and the
    optician said the 9.2 lens seemed tight. Looking back, I would expect
    it to have been less tight than my current 8.6.

    I've always had slightly raised micro veins all around the eye. Could
    that be a result of lack of oxygen or the wrong base curve

    George Bray, Jul 24, 2004
  9. Keith Morris

    Dr Judy Guest

    Soft lenses base curves do not match the cornea, the base curve is chosen
    that fits best, usually quite a bit flatter than the curve of the cornea.
    Different brands of lenses fit differently and one brand may need to be
    flatter or steeper than another brand to fit the same eye.

    Your old lenses also differed in diameter which affects fit: the larger the
    diameter, the flatter the base curve must be. If they were non disposable
    fit 20 years ago, they were likely thicker than today's lenses, had a
    different water content and likely had less oxygen transmission. Those
    factors also affect fit and base curve selection. Older lenses were
    designed to move more with the blink, newer lenses drape more and do not
    move as much

    So, short answer: the 9.2/ 15.0 lens was equivalent to the 8.6/ 14.0 lens
    and your eye did not change.
    Limbal injection can indicate insufficient oxygen to the cornea; this has
    many causes including tight lens, low oxygen lens transmission, high oxygen
    demand cornea.

    Dr Judy
    Dr Judy, Jul 25, 2004
  10. Keith Morris

    George Bray Guest

    Thank you for your very helpful response. I noticed that the diameter
    of my standard non-disposable lens (different brands I believe, and
    design trends I assume) was changed over the years. This seems odd, as
    I would have expected the diameter to reflect the relatively fixed
    size of my pupils or something like that. Perhaps not. A little
    knowledge can be a dangerous thing, especially if you are thinking or
    ordering lens over the Internet and tinkering with the prescription,
    like trying a flatter curve for greater comfort, and lower power to
    help with the onset of long sight. Then I could just accept the 'one
    size fits all' diameter offered by some lens companies, and suddenly I
    have a totally different, totally untried, self-written

    Is there anything which can be done to reverse the very long term
    presence of red veins on the whites of my eyes?

    George Bray, Jul 25, 2004
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